The present invention relates to a method for synchronizing a receiver with a transmitter; in particular, within a mobile radio network.
The rapid technical development in the field of mobile communications has led in recent years to the development of new mobile radio systems of the third generation. An essential role is played in this case by the so-called UMTS (Universal Mobile Telecommunications System), which is based at least partly on the WCDMA (Wideband Code Division Multiple Access) technology. The air interface of this system, UTRA (UMTS Terrestrial Radio Access), forms a central element in this system. This air interface can be implemented in accordance with the prior art via two different types of duplex; TDD (Time Division Duplex) and FDD (Frequency Division Duplex), respectively.
For the purpose of synchronizing a receiver (mobile station, subscriber station) with a transmitter (base station), it is known to transmit a first synchronization signal (synchronization sequence, synchronization code, code word) PSC (Primary Synchronization Code) for detecting a cell and/or a base station, and second synchronization signals (synchronization sequence, synchronization code, code word) SSC (Secondary Synchronization Codes) for the purpose of identifying different parameters of the detected cell and/or base station from the transmitter (base station) to the receiver (mobile station). Such a synchronization is also called cell search.
It is also known in this case from [1] and [5] to use the same PSC and the same SSCs for UTRA FDD and UTRA TDD.
However, the situation arises in this case that for the purpose of synchronization with subscriber stations UTRA FDD requires the use of 16 different SSCs, whereas for the purpose of synchronization with subscriber stations UTRA TDD requires the use of only different SSCs.
The set of 16 SSCs is grouped in UTRA TDD into the five code sets, of which four code sets, which each include three SSCs, are used in UTRA TDD for synchronization, and one code set, which includes four SSCs, is not used in UTRA TDD for synchronization. Three SSCs of a code set are emitted for synchronization purposes in parallel with the PSC in the time slots to which a PSCH (Primary Synchronization Channel) is assigned.
The PSC is a so-called “Generalised hierarchical Golay Sequence” with good a periodic autocorrelation properties that is known per se from [2].
The PSC is accordingly defined by the following construction rule:                Let a=<x1, x2, x3, . . . , x16>=<1, 1, 1, 1, 1, 1, −1, −1, 1, −1, 1, −1, 1, −1, −1, 1> be a sequence of 16 elements.        The PSC is then generated by modulating “a” with the aid of a complementary Golay sequence. The sequence a is therefore repeated, and with each repetition all the elements of the sequence a are multiplied by the value, corresponding to the repetition, of the complementary Golay sequence. Subsequently, all the elements are multiplied by the complex number (1+j). This generates a complex sequence that has identical real and imaginary parts.        
The PSC Cp is therefore defined as:                Cp=<y(0), y(1), y(2), . . . , y(255)>, wherein it holds that:        y=(1+j)×<a, a, a, −a, −a, a, −a, −a, a, a, a, −a, a, −a, a, a>; the value with the smallest index y(0) corresponds in this case to the first symbol or chip transmitted in a time slot.        
The 16 SSCs {C0, . . . , C15}, which are likewise known from [5], are based on Hadamard sequences that are formed by every 16th row, starting with row 0, of a positionally scrambled Hadamard matrix H8. They are likewise complex sequences that have identical real and imaginary parts.
In particular, the 16 SSCs are formed as follows:                The 16 SSCs (SSC code words) {C0, . . . , C15} can be obtained by a positional multiplication of a Hadamard sequence by the sequence z that is defined as        z=<b, b, b, −b, b, b, −b, −b, b, −b, b, −b, −b, −b, −b, −b>, wherein it holds that:        b=<x1, . . . , x8, −x9, . . . , −x16>=(1, 1 −1, 1, 1, 1, −1, −1, −1, 1, −1, 1, −1, 1, 1, −1>;        The Hadamard sequences are defined as rows of the matrix H8, H8 being determined by the following recursive definition:        
            H      0        =          (      1      )                          H        k            =              (                                                            H                                  k                  -                  1                                                                                    H                                  k                  -                  1                                                                                                        H                                  k                  -                  1                                                                                    -                                  H                                      k                    -                    1                                                                                      )              ,          k      ≥      1                      The rows are enumerated from the top down, starting with 0 for the first row (that is, the row containing only ones).        The nth Hadamard sequence is now defined as the nth row of H8, the rows being enumerated in sequence from the top down with n=0, 1, 2, . . . , 255.        Let hm(i) and z(i) respectively be the ith symbol of the sequence hm and z, respectively, wherein i=0, 1, 2, . . . , 255, and wherein i=0 refers to the symbol recorded furthest left.        The ith SCH code word, CSCH,i, wherein i=0, . . . , 15 is then defined as CSCH,i=(1+j)×<hm(0)×z(0), hm(1)×z(1), hm(2)×z(2), . . . , hm(255)×z(255)>, wherein m=(16×i) and the symbol recorded furthest left corresponds to the symbol or chip that is first emitted.        Such an SCH code word is defined for each 16th row of the matrix H8; this yields a total of 16 different SCH code words.        The SSCs, {C0, . . . , C15}, are now defined by these SCH code words, CSCH,i, as: Ci=CSCH,i, i=0, . . . , 15.        
1. The second synchronization sequences are also denoted below with the aid of SSCi or SSCi, wherein it holds that:
SSCi=SSCi=Ci=CSCH,i, i=0, . . . , 15;
Since, now, one PSC and three SSCs of a code set are emitted in parallel for synchronization purposes, and correlation calculations are carried out at the receiving end for the purpose of synchronization, the grouping of the set of the SSCs to form code sets has an influence on the quality of and the outlay on these correlation calculations, and thus the synchronization or the cell search.
An improved grouping of SSCs to form used code sets is proposed in [5], in which the grouping was determined simply with the aid of the sequence of the SSCs:
Code set 1: SSC0, SSC1, SSC2 
Code set 2: SSC3, SSC4, SSC5 
Code set 3: SSC6, SSC7, SSC8 
Code set 4: SSC9, SSC10, SSC11 
The following grouping of SSCs to form used code sets is proposed in [1], in which the grouping was performed using the following rules:
a) select as used SSCs the 12 SSCs from the possible 16 that have the smallest RMS (Root Mean Square) value of the cross correlation relative to the PSC. The RMS value in this case denotes the root of the mean square of the CCF (cross-correlation function) of the SSC's with the PCS. This rule is based on the following finding: if a high cross correlation exists between an SSC and the PCS, it is possible that, in the search for the PSC that is typically carried out by a correlation of the received signal with the PSC, the mobile station could erroneously declare such a high cross correlation with the SSC as PSC.
b) These 12 SSCs are grouped into code sets in such a way that the mean RMS value for all three SSCs located in a code set is also minimized for the worst group.
The following grouping of SSCs to form used code sets resulted in [1] from the application of these criteria:
Code set 1: SSC5, SSC8, SSC11 
Code set 2: SSC0, SSC1, SSC15 
Code set 3: SSC12, SSC13, SSC14 
Code set 4: SSC4, SSC6, SSC10.
However, as will be set forth later on, this selection is not optimal.
The present invention is directed toward specifying a method for synchronizing a receiver with a transmitter, and a method for cell search that permits reliable synchronization.